Puppies can also get from eating their own poop, that's how Spawn got his. He could've also seen his mother doing it when he was younger. It is very common in puppies. When I first got him he did that when he had his accident in the house, now I keep him away from it once he's outside and pick it up and throw it out right away. Here is a link where you can read about it.
Coccidia (Coccidiosis) in Dogs: A Cause of Diarrhea
Sorry, new to this, so I really don't know how to post a link yet.
Mayra